Everyone knows the trick for determining whether a number is divisible by 9, but this is just a baby case of an old trick called Casting out nines that people have long used to double check their arithmetic. Casting out nines won’t guarantee that you did the problem correctly, but it’ll catch most of the common arithmetic errors.
But how does it work? Are there other tricks to check divisibility by other numbers? Indeed there are, and you can learn all about them at the next meeting of the Fayetteville Math Circle: Saturday, September 6th, 2pm at the Fayetteville Public Library.
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